Interferometer module

ABSTRACT

The invention relates to a differential interferometer module adapted for measuring a direction of displacement between a reference mirror and a measurement mirror. In an embodiment the differential interferometer module is adapted for emitting three reference beams towards a first mirror and three measurement beams towards a second mirror for determining a displacement between said first and second mirror. In a preferred embodiment the same module is adapted for measuring a relative rotation around two perpendicular axes as well. The present invention further relates to a lithography system comprising such a interferometer module and a method for measuring such a displacement and rotations.

BACKGROUND

The invention relates to an interferometer module adapted for measuringa displacement between a reference mirror, for instance provided on theoptical column of an exposure tool, and a measurement mirror, forinstance provided on a target carrier of the exposure tool which ismoveable relative to the optical column. The invention further relatesto lithography system comprising such an interferometer module, and amethod for measuring such a displacement.

U.S. Pat. No. 7,224,466 provides a compact differential interferometerfor measuring a displacement between a measurement mirror and areference mirror along two axes. The interferometer uses sharedmeasurement and reference beams that respectively reflect frommeasurement and reference mirrors before that shared beams are splitinto individual beams corresponding to the measurement axes of theinterferometer. By essentially reflecting the measurement beam andreference beam twice in the respective measurement mirror and referencemirror, the beam path is extended and the resolution of theinterferometer improved. A drawback of the known interferometer moduleis that it not possible using said module to unambiguously determine adirection of a displacement between said measurement mirror and saidreference mirror, i.e. whether they are brought closer together orfarther apart.

It is an object of the present invention to provide a interferometermodule allowing determination of a direction of displacement between ameasurement mirror and a reference mirror.

SUMMARY OF THE INVENTION

According to a first aspect, the present invention provides aninterferometer module adapted for emitting a measurement beam and anassociated reference beam towards a respective measurement mirror andreference mirror, said interferometer module comprising: a beam combinerfor combining a reflection of said measurement beam and said referencebeam into a combined beam, a non-polarizing beam splitter for splittingsaid combined beam into a first split beam and a second split beam, eachsplit beam comprising a component of said reference beam and saidmeasurement beam, a first polarizing beam splitter, for splitting saidfirst split beam into a first polarized beam having a first polarizationand a second polarized beam having a second polarization, a secondpolarizing beam splitter, for splitting said second split beam into athird polarized beam having a third polarization and a fourth polarizedbeam having a fourth polarization, a first, second, third and fourthdetector for detecting a beam energy of said first, second, third andfourth polarized beams respectively, wherein said polarizations of saidfirst, second, third and fourth polarized beams are differentpolarizations. The four detectors provide four interference signalswherein each interference signal is typically a sinusoid signal shiftedin phase relative to the other signals. It is thus possible to determinea direction of displacement between said measurement mirror and saidreference mirror. Moreover, by using interpolating methods between thefour signals, the resolution of the measurement may be improved. Thecoherent beams are internally coherent but are not necessarily coherentwith respect to each other. The combined beams are formed by reflectedreference and associated reflected measurement beams, which at leastpartially coincide at their corresponding beam detector.

In an embodiment the interferometer module is further adapted foremitting two additional measurement beams and two correspondingadditional reference beams towards said measurement mirror andrespective reference mirror, wherein said beam combiner is furtheradapted for combining reflections of said additional measurement beamsand said corresponding additional reference beams into additionalcombined beams substantially parallel to said combined beam, whereinsaid non-polarizing beam splitter is arranged for splitting saidadditional beams into additional first split beams and additional secondsplit beams, and wherein said first and second polarizing beam splittersare arranged for splitting said additional first split beams and saidadditional second split beams respectively into additional first,second, third and fourth polarized beams directed onto additional first,second, third and fourth detectors. Preferably, the additionalmeasurement beams comprise two beams parallel to said measurement beam,wherein said measurement beam and additional measurement beams areemitted non-coplanarly towards the measurement mirror. Likewise, theadditional reference beam preferably comprise two beams parallel to saidreference beam, wherein said reference beam and additional measurementbeams are emitted non-coplanarly towards the reference mirror. Thus themeasurement beams may have define two substantially perpendicularplanes, and the reference beams as well. Using such a module, it ispossible to determine a relative displacement between said referencemirror and said measurement mirror along at least three measurementaxes. From the detected beam energies a displacement along a direction xalong the measurement beams may be determined, together with a rotationaround axes Ry and Rz which are perpendicular to each other and to saiddirection x. As the same beam non-polarizing beam splitter is used tosplit all combined beams into said first split beam and second splitbeams, there is no need for aligning several different such beamsplitters for each combined beam.

In an embodiment the interferometer module further comprises a blockingelement comprising three irises, said blocking element arranged betweensaid beam combiner and said non-polarizing beam splitter for partiallyblocking said combined beam and said two additional combined beamsrespectively. The irises function to filter out so calledJung-frequencies, and/or ensure that only center portions of eachcombined beam reach the non-polarizing beam splitter.

In an embodiment said blocking element comprises silicon, which isrelatively insensitive to thermal deformation as compared to forinstance aluminum. Preferably, the blocking element is manufacturedusing lithographic techniques.

In an embodiment said first polarizing beam splitter is rotated by 45degrees relative to said second polarizing beam splitter.

In an embodiment said first polarizing beam splitter is adapted forsplitting said first split beam such that said first polarized beam hasa parallel polarization and said second polarized beam has aperpendicular polarization, and wherein said second polarizing beamsplitter is adapted for splitting said second split beam such that saidthird polarized beam has a 45 degree polarization and said fourthpolarized beam has a 135 degree polarization.

In an embodiment said non-polarizing beam splitter is directly adjacentto said first polarizing beam splitter and/or to said second polarizingbeam splitter.

In an embodiment said beam combiner is fixedly attached to saidnon-polarizing beam splitter, forming a macro element. Such amacro-element less is sensitive to vibrations and misalignment duringtransport than when said beam combiner and non-polarizing beam splitterare separate elements.

In an embodiment said non-polarizing beam splitter is fixedly attachedto said first polarizing beam splitter and/or to said second polarizingbeam splitter using an optically neutral adhesive. The non-polarizingbeam splitter and the first and second polarizing beam splitters maythus be formed as a macro-element. Besides a lower sensitivity tovibrations and/or misalignment, the use of such an optically neutraladhesive reduces the loss of light which occurs when a light beamtraverses multiple interfaces between materials having differentrefractive indices.

In an embodiment said reference mirror is fixedly attached within saidinterferometer module. The reference beams thus remain within themodule, while the measurement beams are emitted to a measurement mirrorexterior to the module. The interferometer module is thus adapted formeasuring a signal indicative of a displacement between said measurementmirror and said interferometer module.

In an alternative embodiment said interferometer module is adifferential interferometer module, further comprising: a beam sourceadapted for providing three coherent beams, a beam splitter unit adaptedfor splitting said three beams into respective pairs of measurementbeams and associated reference beams, wherein the three measurementbeams are incident on a first mirror, and wherein the three referencebeams are incident on a second mirror moveable with respect to saidfirst mirror, wherein said beam combiner is arranged for combining eachreflected measurement beam with its associated reflected reference beamto a combined beam of said combined beams. The module emits saidreference beams and said measurement beams to said reference mirror andmeasurement mirror respectively, i.e. both said reference beams and themeasurement beams are emitted to a location outside of the module. Thedifferential interferometer module is adapted for measuring adisplacement between said measurement mirror and said reference mirroralong three non-coplanar measurement axes. It is thus possible, using asingle interferometer module, for instance to determine a relativedisplacement between said mirrors along three different measurementaxes.

In an embodiment said beam splitter unit comprises a single beamsplitter for splitting said three beams into three measurementbeam/reference beam pairs.

In an embodiment said beam detectors each comprise a beam intensitydetector or a beam energy detector for detecting an intensity or energyof a corresponding combined beam. Alternatively, the beam detectors mayeach comprise a light detector for detecting the optical power, orenergy, of a beam emitted thereon.

In an embodiment the beam splitter unit is adapted for emitting saidthree measurement beams non-coplanarly, and/or for emitting said threereference beams non-coplanarly. The interferometer module according tothe invention thus makes it possible to determine a displacement betweensaid mirrors along a direction x, and a rotation around directions Rzand Rz which are perpendicular to each other and to direction x.

In an embodiment a first incident measurement beam and a second incidentmeasurement beam span a first plane and the second incident measurementbeam and a third incident measurement beam span a second plane at anangle α to the first plane, and a first incident reference beam and asecond incident reference beam span a third plane and the secondincident reference beam and a third incident reference beam span afourth plane at substantially the same angle α to said third plane.

In an embodiment said angle α is 90°.

In an embodiment the second plane and the fourth plane substantiallycoincide.

In an embodiment the three incident measurement beams are substantiallyparallel to each other and/or the three incident reference beams aresubstantially parallel to each other.

In an embodiment each of said three incident measurement beams issubstantially parallel to its associated incident reference beam.

In an embodiment said first and second mirror are spaced apart from saidmodule.

In an embodiment the inside of said module is substantially filled witha solid material, preferably a cured epoxy-resin, more preferablyStycast®. As the optical elements of the module are thus securely heldin place, the module is less susceptible to alignment errors due tovibrations or handling.

In an embodiment the beam splitter and the beam combiner are comprisedin a single integrated unit.

According to a second aspect the present invention provides aninterferometer module comprising: a beam source adapted for providingthree coherent beams, a beam splitter adapted for splitting said threebeams into respective pairs of measurement beams and associatedreference beams, wherein the three measurement beams are incident on afirst mirror, and wherein the three reference beams are incident on asecond mirror moveable with respect to said first mirror, wherein saidbeam combiner is arranged for combining each reflected measurement beamwith its associated reflected reference beam to a combined beam, and ahousing, wherein said beam splitter and said beam combiner are arrangedwithin the housing, and said housing a first set of three holes forallowing passage of said three measurement beams and said threereflected measurement beams from within to housing to outside saidhousing and vice versa, and a second set of three holes for allowingpassage of said three reference beams from within said housing tooutside said housing and vice versa.

According to a third aspect the present invention provides a lithographysystem comprising an interferometer module according to any one of thepreceding claims, said system further comprising: an optical column forprojecting a pattern onto a target, a target carrier for moving saidtarget relative to the optical column, a controller, for controllingmovement of said target carrier relative to the optical column, whereinthe target carrier is provided with a first mirror, and wherein theoptical column is provided with a second mirror, wherein saidinterferometer is arranged for emitting said measurement beam on saidfirst mirror and said reference beam on said second mirror, wherein saidcontroller is adapted for controlling movement of said target carrierrelative to said optical column based on the energies of beams detectedby said first, second, third and fourth detectors. The lithographysystem is preferably a lithography system adapted for projecting saidpattern onto said target during movement of the target carrier relativeto the projection optics. More preferably, the lithography system is acharged particle lithography system adapted for projecting a pluralityof charged particle beamlets onto said target.

According to a fourth aspect, the present invention provides a method ofdetermining a displacement of a measurement mirror relative to areference mirror using an interferometer module according to any one ofthe preceding claims, said method comprising the steps of: measuring theenergies of said first, second, third and fourth polarized beams at saidfirst, second, third and fourth detector respectively, providing a firstdisplacement signal as a difference between said first and secondmeasured energy, and providing a second displacement signal as adifference between said third and fourth measured energy. Alternatively,instead of measuring the energies, beam intensities may be measured.

The various aspects and features described and shown in thespecification can be applied, individually, wherever possible. Theseindividual aspects, in particular the aspects and features described inthe attached dependent claims, can be made subject of divisional patentapplications.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be elucidated on the basis of an exemplary embodimentshown in the attached drawings, in which:

FIG. 1A schematically shows a cross-sectional side view of adifferential interferometer module according to the invention,

FIG. 1B shows a graph of signals obtained using a differentialinterferometer module of FIG. 1A,

FIG. 2A schematically shows a cross-sectional side view of adifferential interferometer module similar to that of FIG. 1A, with theoptical elements of the lower section adjacent to each other,

FIG. 2B schematically shows a cross-sectional side view of adifferential interferometer module similar to that of FIG. 1A, in whichthe optical elements of the lower section are attached each other usingan optically neutral adhesive,

FIGS. 3A and 3B schematically show embodiments in which all opticalelements are adjacent, forming a single unit,

FIG. 4 schematically shows an embodiment of the invention for emittingthree non-coplanar measurement beams and corresponding reference beams,

FIG. 5 schematically shows an embodiment of the invention, furthercomprising two beam path adjusters, for changing the mutual distancesbetween the reference beams and the measurement beams respectively,

FIGS. 6A and 6B show schematic side views of a lithography systemaccording to the present invention,

FIG. 6C shows a schematic side view of a further embodiment of alithography system according to the present invention,

FIGS. 7A and 7B show a schematic front view and an isometric viewrespectively of a differential interferometer module according to thepresent invention,

FIGS. 8A and 8B show a cross-sectional side view and a cross-sectionaltop view a an differential interferometer module according to theinvention,

FIGS. 9A and 9B show a top view and a side view respectively of alithography system comprising two interferometer modules according tothe present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A shows a detail of a preferred embodiment of an interferometermodule 100 according to the present invention. A single coherent beam bis emitted onto polarizing beam splitter 101, which splits the beam binto a polarized measurement beam Mb and an associated polarizedreference beam Rb. After having passed the polarizing beam splitter 101,the measurement beam Mb passes a quarter wave plate 103. The incidentmeasurement beam is then reflected back by first mirror 21, and againpasses the quarter wave plate 103. Subsequently the reflectedmeasurement beam is reflected through an iris 140 by the polarizing beamsplitter 101.

Similarly, the part of the coherent beam that forms the reference beamRb is reflected by prism 102 through a quarter wave plate 103 andincident on second mirror 81. The reference beam Rb is then reflectedback by mirror 81 and again passes through the same quarter wave plate103, after which it is reflected by prism 102, through polarizing beamsplitter 101 towards iris 104.

Thus, when the interferometer module is active, a combined beam Cbpasses the iris 104. A non-polarizing beam splitter 105 splits thecombined beam up into two, wherein the two combined beam portions intowhich the combined beam is split up comprise both a portion of thereflected reference beam and a portion of the reflected measurementbeam. The two beam portions in turn are split up by polarizing beamsplitters 106 and 107 respectively. The polarizing beam splitter 106 isrotated 45 degrees with respect to polarizing beam splitter 107. Thusfour distinct combined beam portions result, having a parallelpolarization, a perpendicular polarization, a 45 degree polarization anda 135 degree polarization respectively. Detectors 108,109,110 and 111,convert intensities of these four combined beam portions into a firstsignal sig1, a second signal sig2, a third signal sig3 and a fourthsignal sig4 respectively. The beam splitter and the beam combiner arearranged within a housing 130 of the module.

FIG. 1B shows a graph of a difference between said signals sig1 andsig2, and of a difference between said signals sig3 and sig4 as a targetcarrier, for instance comprising the measurement mirror, is moved at aconstant velocity with respect to the projection optics, which forinstance comprises the reference mirror. The graph shows two sinusoidalcurves 121, 122 that are used to determine a wafer table displacementand thus the wafer table position.

When only a single sinusoid curve is available, it may be difficult todetermine a direction of relative movement when a change in intensityfrom a peak level to a lower level occurs, as both movement of the wafertable towards and away from the optical column will result in a lowerintensity signal. According to the present invention, a direction ofmovement can be determined at any time by using two sinusoid curves thatare out of phase with respect to each other, for instance out of phaseby 45 degrees. A further advantage of using two curves instead of one isthat measurements may be carried out more accurately. For instance, whena peak is measured for curve 121, a small movement to either side willresult in a small change in measured intensity signal of the curve.However, the same small movement results in a large change in measuredintensity signal of curve 122, which may then be used to determine thedisplacement instead.

FIGS. 2A and 2B schematically show interferometer module or head 200according to the invention similar to the embodiment shown in FIG. 1A.However, in FIG. 2A, non-polarizing beam splitter 105 is arrangeddirectly adjacent to polarizing beam splitters 106 and 107, such thatthere are fewer interfaces between materials with different refractiveindices when compared to FIG. 1A, resulting in reduced loss of light.

In FIG. 2B, non-polarizing beam splitter 105 is attached to polarizingbeam splitters 106 and 107 by means of a layer of an opticallytransparent adhesive 121 and 122 respectively, here shown having anexaggerated width. This embodiment offers the additional advantage thatbeam splitters 105,106,107 with each other is less susceptible to becomemisaligned, for instance due to vibrations or handling.

FIG. 3A schematically shows an interferometer module 300 according tothe invention, in which the optical elements are all adjacent.Non-polarizing beam splitter 105 is adjacent to quarter wave plate 103between said beam splitter 105 and polarizing beam splitter 101. Nonpolarizing beam splitter 105 is further adjacent to polarizing beamsplitters 106 and 107. This embodiment is particularly easily assembled,as alignment of the optical elements with each other is achieved byarranging the optical elements such that they are adjacent, i.e. abutalong planar surfaces.

In FIG. 3B a fixed connection between non-polarizing beam splitter 105and its neighboring optical components is provided by layers 121,122,123of an optically transparent adhesive.

FIG. 4 schematically shows an interferometer module or head 400according to the invention similar to the embodiment shown in FIG. 1A,however wherein three coherent light beams b1,b2,b3 are incident onpolarizing beam splitter 101 instead of only one. This results in threereference beams rb1,rb2,rb3 being emitted towards the second mirror 81,and three measurement beams being emitted towards the first mirror 21.The three reference beams and associated three measurement beams areemitted non-coplanarly from the module. The housing 130 is to this endprovided with two sets of three holes each, the first set allowingpassage of the three measurement beams and the second set allowingpassage of the three reference beams from the

The three reflected reference beams and associated three reflectedmeasurement beams are combined into three combined beams which pass theiris 104 and are split up in the same manner as described above. Beamreceiving intensity detectors 108 ₁,108 ₂,108 ₃ detect an interferenceof a portion of each of the combined beams cb1,cb2,cb3 respectively.Detectors 109 ₁,109 ₂,109 ₃, 110 ₁,110 ₂,110 ₃, 111 ₁,111 ₂,111 ₃function likewise for combined beam portions with differentpolarizations, resulting in a total of 12 detection signals. From thesedetection signals sinusoidal curves can be constructed which provideinformation on relative displacement and rotation of the two mirrors81,21.

FIG. 5 schematically shows an interferometer module or head according tothe invention similar to the embodiment shown in FIG. 4. However,instead of iris 104, a blocking element 140 is provided with threeseparate irises 141,142,143 for letting through only a center portion ofrespective combined beams cb1,cb2,cb3. Moreover, the embodiment of FIG.5 is provided with beam path adjusters 181,182 which respectively adjustthe mutual distances between reference beams rb1,rb2 and rb3, and themutual distances between measurement beams mb1,mb2 and mb3. The beampath adjusters 181,121 each comprise a number of internal reflectionprisms for adjusting the path of the reference beams rb1,rb2,rb3 andmeasurement beams mb1,mb2,mb3 respectively, wherein the beam pathadjusters are provided with separate prisms for each beam.Alternatively, the beam path adjusters may comprise two mirrors for eachbeam to achieve a similar effect. In an embodiment (not shown), the beampath adjusters 121 and/or 181 comprise Fresnel rhombs arranged to act asa quarter wave plate. In this last embodiment, both quarter wave plates103 facing mirrors 81 and 21 may be omitted. It is noted that althoughFIGS. 4 and 5 schematically depict the beams b1,b2 and b3 coplanarly, inreality the three beams are parallel yet non-coplanar, and the resultingreference and measurement beams are emitted non-coplanarly onto theirrespective mirrors. Likewise, in the ireses 141,142,142 are typicallyarranged in an L-configuration on the blocking element 140.

FIG. 6A shows a lithography system 1 according to the present invention.The system comprises a frame 4, to which an optical column 36 having anoptical axis 37 is mounted. The optical column is adapted for projectinga plurality of exposure beamlets 10 onto a target 7. By selectivelyswitching selected exposure beamlets on or of, an exposure surface ofthe target below the optical column may be patterned. The target isplaced on a wafer table 6, which in turn is placed on a chuck 66 whichis moveable with respect to the optical column 36 by means of a stage 9on which the chuck 66 is placed. In the embodiment shown, the chuck,wafer table and stage form a target carrier for moving the target 7relative to the optical column 36.

The chuck 66 comprises a first mirror 21, comprising a substantiallyplanar surface at substantially the same level or height within thesystem as the target 7 or exposure surface thereof. The optical columncomprises a second mirror 81, which comprises a substantially planarsurface close to the projection end of the optical column.

The system further comprises a modular interferometer head 60, ordifferential interferometer module, which is mounted to the frame 4 bymeans of a kinematic mount 62,63,64. The modular interferometer head 60emits reference beams Rb onto the second mirror 81, and associatedmeasurement beams Mb onto the first mirror 21. Though not shown in thisfigure, the reference beams comprise three reference beams, and themeasurement beams comprise three measurement beams, and a relativemovement between the first mirror 81 and second mirror 21 is measured byevaluating an interference between a reference beam and its associatedmeasurement beam.

The three measurement beams Mb and the three reference beams Rboriginate from a laser unit 31 which supplies a beam of coherent light,and which is coupled into the interferometer module 60 via an opticalfiber 92 which forms part of a beam source for the module 60.

FIG. 6B schematically shows the lithography system 1 of FIG. 6A, whereinthe lithography system comprises a vacuum housing 2. Within the vacuumhousing 2, only the interferometer head 60 and its connections, andfirst 81 and second mirrors 21 are shown, though it will be understoodthat the target carrier of FIG. 6A will be contained within the vacuumchamber 2 as well.

The optical fiber 92 from laser 31 passes through a wall of said vacuumchamber 2 through a vacuum feed-through 91. Signals representative ofinterference between measurement beams and their associated referencebeams are transported from the interferometer module 60 out of thevacuum chamber 2 via signal wires 54, which pass through vacuumfeed-through 61.

FIG. 6C schematically shows a lithography system similar to the systemshown in FIG. 1A, wherein the system is a charged particle beamlithography system comprising electron optics 3 for providing aplurality of charged particle beamlets, and wherein the projectionoptics 5 comprise a plurality of electrostatic lenses for individuallyfocusing said charged particle beamlets onto an exposure surface of thetarget 7. The projection optics comprises actuators 67 for adjusting anorientation and/or position of the projection optics relative to theframe 4. The system further comprises a signal processing module 94adapted providing a position and/or displacement signal to a stagecontrol unit 95 for controlling movement of a stage 11. Signals aretransmitted from the interferometer module 60 and the alignment sensor57 via signal wires 54,58 which pass through vacuum feed-throughs 61 and59, to the signal processing module 94, which processes these signals toprovide a signal for actuating the stage 11 and/or the projection optics5. The displacement of the wafer table 6, and thus of the target 7supported thereby relative to projection optics 5 is thus continuouslymonitored and corrected.

In the embodiment shown, the wafer table 6 is supported by a moveablestage 11 via a kinematic mount 8, and the stage 9 may be moved relativeto the projection optics 5 in a direction towards or away from theinterferometer module 60. The differential interferometer module 60emits three reference beams towards a mirror on the projection optics,and emits three measurement beams towards a mirror on the wafer table.

FIGS. 7A and 7B shows a front view and an isometric view respectively ofthe interferometer module of FIG. 6A. The interferometer module 60comprises a kinematic mount 62,63,64 for easy and highly precisealignment of the module during mounting of the module on the frame. Theinterferometer module comprises three holes 71,72,73 for emitting threecorresponding reference beams rb1,rb2,rb3, as well as for receivingreflections thereof back into the module. The interferometer modulefurther comprises three holes 74,75,76 for emitting three correspondingmeasurement beams mb1,mb2,mb3, as well as for receiving reflectionsthereof back into the module. Hole 73 for emitting a reference beam islocated at a distance d5 of 4 mm from hole 75 for emitting a measurementbeam. Holes 71 and 72 are spaced apart by a distance d1, holes 72 and 73by a distance d2, holes 74 and 75 by a distance d3 equal to distance d1,and holes 75 and 76 by a distance d4 equal to distance d2. In theembodiment shown the distances d1,d2,d3,d4 and d5 are center-to-centerdistances equal to 12, 5, 12, 5 and 4 millimeter respectively. In FIG.2B in can be seen that the first reference beam rb1 and second referencebeam rb2 span a first plane, and the second reference beam rb2 and thirdreference beam rb3 span a second plane, wherein the second plane is atan angle α (not shown) of 90 degrees with respect to the first plane.Likewise, the first measurement beam mb1 and second measurement beam mb2span a third plane, and the second measurement beam mb2 and thirdmeasurement beam mb3 span a fourth plane, wherein the third plane is atsubstantially the same angle α (not shown) with respect to the fourthplane.

FIGS. 8A and 8B show a schematic side view and top view respectively ofan embodiment of the differential interferometer module 60 according tothe present invention. The module comprises a primary beam splitter unit32,33,34, for splitting a laser beam LB emitted by laser unit 31 up intothree coherent light beams b1,b2,b3. The primary beam splitter unitshown is a unit comprising two beam splitters 32,34 and two reflectingprisms 33,35. Each of the coherent light beams b1,b2,b3 are then emittedtoward a secondary beam splitter unit 42,43, adapted for splitting saidthree coherent light beams b1,b2,b3 up into respective measurement andassociated reference beam pairs. The first of these pairs comprisesmeasurement beam rb1 and associated reference beam rb1, the second ofthese pairs comprises measurement beam rb2 and associated reference beamrb2, and the third pair comprises measurement beam rb3 and associatedreference beam rb3.

Thus 6 beams are emitted from the secondary beam splitter unit, threereference beams rb1,rb2,rb3 and three associated measurement beamsmb1,mb3,mb3.

The reference beams rb1,rb2,rb3 are emitted incident on second mirror 81of the optical column, while the measurement beams mb1,mb2,mb3 areemitted incident on first mirror 21 of the target carrier. The referenceand measurement beams are reflected back into the module 60, inparticular back into secondary beam splitter unit 42,43, which acts as abeam combiner 42,43 for the reflected measurement beams and theirassociated reference beams. The beam combiner thus emits three combinedbeams cb1,cb2,cb3, wherein each of said combined beams is formed by areflected measurement beam and its associated reference beam at leastpartially overlapping at corresponding light receivers 51,52,53 or beamdetectors, in this case light intensity detectors 51,52,53 comprisingphoto-diodes. A changing interference of the measurement beams andassociated reference beams at any of the beam receivers results in achange in the light intensity at that beam receiver. The photo-diodesconvert a light intensity signal to an electrical signal, which is fedout of the module 60 unamplified.

FIGS. 9A and 9B show a top view and a side view of a lithography systemaccording to the present invention, in which a first and a seconddifferential interferometer module 60A,60B as described herein arearranged for measuring a displacement of the wafer 7 relative toprojection optics 5. The projection optics is provided with two planarmirrors 81A, 81B, arranged at a 90 degrees angle with respect to eachother. The wafer 7 is supported by a wafer table 6 which comprises twoplanar mirrors 21A and 21B arranged at a 90 degrees angle with respectto each other as well. The first differential interferometer module 60Aemits three reference beams rb1,rb2,rb3 on mirror 81A of the projectionoptics, and emits three measurement beams on mirror 21A of the wafertable. Similarly, the second differential interferometer module 60Bemits reference beams on mirror 81B of the projection optics, and emitsmeasurement beams on mirror 21B of the wafer table.

In summary the present invention relates to a differentialinterferometer module adapted for measuring a direction of displacementbetween a reference mirror and a measurement mirror. In an embodimentthe differential interferometer module is adapted for emitting threereference beams towards a first mirror and three measurement beamstowards a second mirror for determining a displacement between saidfirst and second mirror. In a preferred embodiment the same module isadapted for measuring a relative rotation around two perpendicular axesas well. The present invention further relates to a method for measuringsuch a displacement and rotations.

It is to be understood that the above description is included toillustrate the operation of the preferred embodiments and is not meantto limit the scope of the invention. From the above discussion, manyvariations will be apparent to one skilled in the art that would yet beencompassed by the spirit and scope of the present invention.

IN THE FIGURES

-   LB laser beam-   b coherent beam-   cb combined beam-   cb1,cb2,cb3 combined beams-   b1,b2,b3 coherent beams-   rb1,rb2,rb3 reference beams-   rb reference beams-   mb1,mb2,mb3 measurement beams-   mb measurement beams-   sig1,sig2,-   sig3,sig4 intensity signals-   1 lithography system-   2 vacuum housing-   3 electron optics-   4 frame-   5 projection optics-   6 wafer table-   7 wafer-   8 kinematic mount-   9 stage-   10 plurality of exposure beamlets-   11 stage-   21,21A,21B first mirror-   31 laser unit-   32,34,42 beam splitter-   33,35,43 prism-   36 optical column-   37 optical axis-   51,52,53 light detectors-   54,58 signal wires-   55 electronics of interferometer-   56 measurement of position second mirror with respect to first    mirror-   57 alignment sensor-   59,61 vacuum feed-through-   60, 60A, 60B interferometer head/interferometer module-   62,63,64 kinematic mount-   65 alignment marker-   66 chuck-   67 actuators of projection optics-   71,72,73 holes for measurement beams-   74,75,76 holes for reference beams-   81, 81A,81B second mirror-   91 vacuum feed-through-   92 optical fiber-   94 signal processing module-   95 stage control-   100,200, 300,400 interferometer module/interferometer head-   101 polarizing beam splitter-   102 prism-   103 quarter wave plate-   104 iris-   105 non polarizing beam splitter-   106,107 polarizing beam splitter-   108, 108 ₁, 108 ₂,108 ₃ detectors-   109, 109 ₁, 109 ₂,109 ₃ detectors-   110, 110 ₁, 110 ₂,110 ₃ detectors-   111, 111 ₁, 111 ₂,111 ₃ detectors-   121,122 sinusoidal curves-   130 housing-   140 blocking element-   141,142,143 iris-   121,181 beam path adjuster

1. A lithography system comprising: a laser unit (31) for generating acoherent laser beam (b); a vacuum housing (2); a frame (4) within thevacuum housing (2); an optical column (36) within the vacuum housing (2)mounted to the frame (4) for projecting a pattern onto a target, whereinthe optical column (36) is provided with a first mirror (21); a targetcarrier arranged within the vacuum housing (2) for moving said targetrelative to the optical column (36), wherein the target carrier isprovided with a second mirror (81); a differential interferometer module(60) for receiving said coherent laser beam (b), the interferometermodule (60) being arranged for emitting a measurement beam (Mb) on saidfirst mirror (21) and a reference beam (Rb) on said second mirror (81),the interferometer module (60) being configured for providing one ormore output signals related to a relative position between the firstmirror (21) and the second mirror (81), and the system furthercomprising: a controller (95) for controlling movement of said targetcarrier relative to the optical column (36) based upon said one or moreoutput signals, the controller (95) being coupled to the interferometermodule (60) for receiving said one or more output signals, thecontroller being incorporated in the system outside the vacuum housing(2), and an optical fiber (92); wherein said laser unit (31) isincorporated in the system outside the vacuum housing (2), and whereinthe interferometer module (60) is mounted to the frame (4) within thevacuum housing (2) and comprises an input, which input is coupled to thelaser unit (31) via the optical fiber (92).
 2. The lithography systemaccording to claim 1, wherein the controller (95) is coupled to theinterferometer module (60) via respective signal wires (54).
 3. Thelithography system according to claim 1, said interferometer modulecomprising: a beam combiner (101) for combining a reflection of saidmeasurement beam (Mb) and said reference beam (Rb) into a combined beam(Cb), a non-polarizing beam splitter (105) for splitting said combinedbeam into a first split beam and a second split beam, each split beamcomprising a component of said reference beam and said measurement beam,a first polarizing beam splitter (106), for splitting said first splitbeam into a first polarized beam having a first polarization and asecond polarized beam having a second polarization, a second polarizingbeam splitter (107), for splitting said second split beam into a thirdpolarized beam having a third polarization and a fourth polarized beamhaving a fourth polarization, a first, second, third and fourth detector(108, 109, 110, 111) for detecting a beam energy of said first, second,third and fourth polarized beams respectively and for providing said oneor more output signals, wherein said polarizations of said first,second, third and fourth polarized beams are different polarizations,and wherein the controller (95) is configured for controlling movementof said target carrier relative to the optical column (36) based on thebeam energies of beams detected by said first, second, third and fourthdetectors.
 4. The lithography system according to claim 1, wherein saidoutput signals are generated by respective photo-diodes.
 5. Thelithography system according to claim 3, wherein the interferometermodule (60) is further adapted for emitting two additional measurementbeams and two corresponding additional reference beams towards saidmeasurement mirror and respective reference mirror, wherein said beamcombiner is further adapted for combining reflections of said additionalmeasurement beams and said corresponding additional reference beams intoadditional combined beams substantially parallel to said combined beam,wherein said non-polarizing beam splitter is arranged for splitting saidadditional beams into additional first split beams and additional secondsplit beams, and wherein said first and second polarizing beam splittersare arranged for splitting said additional first split beams and saidadditional second split beams respectively into additional first,second, third and fourth polarized beams directed onto additional first,second, third and fourth detectors.
 6. The lithography system accordingto claim 5, wherein said interferometer module (60) further comprises ablocking element comprising three irises, wherein said blocking elementis arranged between said beam combiner and said non-polarizing beamsplitter for partially blocking said combined beam and said twoadditional combined beams respectively.
 7. The lithography systemaccording to claim 6, wherein said blocking element comprises silicon.8. The lithography system according to claim 1, wherein saidinterferometer module further comprises: a beam source adapted forproviding three coherent beams, a beam splitter unit adapted forsplitting said three beams into respective pairs of measurement beamsand associated reference beams, wherein the three measurement beams areincident on a first mirror, and wherein the three reference beams areincident on a second mirror moveable with respect to said first mirror,a housing provided with a first set of three holes for allowing passageof said three measurement beams from within the housing to outside saidhousing and for allowing passage of three reflected measurement beamsfrom outside the housing to within the housing, and a second set ofthree holes for allowing passage of said three reference beams fromwithin said housing to outside said housing and for allowing passage ofthree reflected measurement beams from outside the housing to within thehousing, and a beam combiner is arranged for combining each reflectedmeasurement beam with its associated reflected reference beam to acombined beam, wherein said beam splitter and said beam combiner arearranged within said housing.
 9. The lithography system according toclaim 8, wherein the inside of said interferometer module issubstantially filled with a solid material.
 10. The lithography systemaccording to claim 9, wherein said solid material is a cured epoxyresin.
 11. The lithography system according to claim 8, wherein the beamsplitter and the beam combiner are comprised in a single integratedunit.
 12. Differential interferometer module comprising: a beam sourceadapted for providing three coherent beams; a beam splitter unit adaptedfor splitting said three beams into respective pairs of measurementbeams and associated reference beams, for directing the threemeasurement beams to be incident on a first mirror, and for directingthe three reference beams to be incident on a second mirror moveablewith respect to said first mirror; a beam combiner arranged forcombining each reflected measurement beam with its associated reflectedreference beam to a respective combined beam; a housing provided with afirst set of three holes for allowing passage of said three measurementbeams from within the housing to outside said housing and for allowingpassage of three reflected measurement beams from outside the housing towithin the housing, and a second set of three holes for allowing passageof said three reference beams from within said housing to outside saidhousing and for allowing passage of three reflected measurement beamsfrom outside the housing to within the housing, wherein said beamsplitter unit and said beam combiner are arranged within said housing.13. Differential interferometer according to claim 12, wherein said beamsource comprises an optical fiber (92).
 14. Differential interferometermodule according to claim 12, wherein said three measurements beamscomprise a first, second and third incident measurement beam to beincident on the first mirror, wherein said first and second incidentmeasurement beam span a first plane and wherein the second and thirdincident measurement beam span a second plane at an angle α to the firstplane, wherein said three reference beams comprise a first, second andthird reference beam to be incident on said second mirror, wherein saidfirst and second incident reference beam span a third plane and thesecond incident reference beam and a third incident reference beam spana fourth plane at substantially the same angle α to said third plane.15. Differential interferometer module according to claim 14, whereinsaid angle α is 90°.
 16. Differential interferometer module according toclaim 14, wherein the second plane and the fourth plane substantiallycoincide.
 17. Differential interferometer module according to claim 14,wherein the three incident measurement beams are substantially parallelto each other and/or wherein the three incident reference beams aresubstantially parallel to each other.
 18. Differential interferometermodule according to claim 14, wherein each of said three incidentmeasurement beams is substantially parallel to its associated incidentreference beam.